Michael Faraday discovered magneto-optical (“MO”) effects in 1845. Faraday noticed that magnetic lines of force from a magnet would affect polarized light rays passing through a glass rod. A Scottish scientist named John Kerr later published what came to be known as the Kerr electro-optic effect in 1875. This effect, for which Faraday had searched in vain some 40 years before, is the rotation of the plane of polarization of light in passing through an optical medium across which an electric potential is applied. Kerr's first results were for solid glass; but these were followed by results using liquids in cells. In the following year he published details of another effect, the magneto-optic effect using an electromagnet. The magnetic effect showed that a rotation of the plane of polarization of light occurred upon reflection from the polished pole of a magnet.

While the magneto-optical effects observed by Faraday and Kerr in media such as glass are relatively small, these effects are much larger in magnetic media. More recently, MO effects have been used for a variety of applications including magneto-optical recording (e.g., for high density data storage devices), optical communications, magnetic domain imaging, hysteresis loop plotting, Faraday microscopes, and other applications. For example, with the aid of the MO effects, dynamic processes in, for example, superconductors and magnetic structures in magnetic storage media can be examined. Other applications include, but are not limited to, imaging of electric current values and distributions in integrated circuits, visualization of magnetization dynamics of spin valves, viewing magnetic inks in currency, non-destructive testing of structural metals and imaging in permanent magnets.
For many applications, the preferable magneto-optical media is a film known as magneto-optical indicator film (“MOIF”). See for example Andrae, U.S. Pat. No. 5,583,690; B. Ludescher, et al., “Faraday Low-temperature Microscope for observing Dynamic Magnetization processes in Superconductors (i.e., Faraday-Tieftemperatur-Mikroskop zur Beobachtung dynamischer Magnetisierungsvorgange in Supraeitern”), Laser und Optoelektronik 23 (1991), pages 54–58; L. A. Dorosinskii, et al., Physica C 203 (1992), page 149; and M. V. Indenbohm, et al., Physica C 209 (1993), page 295. A device for detecting magneto-optical anisotropy, particularly of magnetic recording media, is described in U.S. Pat. No. 4,410,227. A laser polarizing microscope for observation of magnetic domains is known from JP 3-185338 (A). A Kerr microscope for examining current paths utilizing the polar Kerr effect is known from German Patent Specification DE 4027049.
Briefly, the magneto-optical Faraday effect causes a rotation of the polarization plane of polarized light by angle φ as it passes through a magneto-optical material of thickness d according to the equation φ=R·M(x)·d, where R represents the material constant (known as Verdet constant) of the magneto-optical material and M(x) represents the magnetization component at point x and parallel to the light path. The rotation of the plane of polarization is visible by observing the light at the polarizer-analyzer intersection. The so-called Kerr microscope uses the Kerr effect, which produces rotation of polarization of light reflected from magneto-optical media.
FIG. 1 shows an example prior art MOIF arrangement used to observe and/or test the magnetic characteristics of a device under test (“DUT”) 4. The MOIF arrangement includes a magneto-optically active layer 1 disposed on a substrate 2. A high reflectivity layer 3 is provided at the interface with device under test (DUT) 4. Incident light 5 passes through the substrate and the active layer 1 to strike the high reflectivity layer 3. The high reflectivity layer 3 reflects the light (6) back toward the substrate 2. The polarization state 7 of the reflected light changes with the magnetic characteristics of device under test 4 due to the Kerr or Faraday magneto-optical effect, and can be observed or measured.
In general with such MOIF implementations, the constant R is so small that the Faraday effect is observed only in special materials such as Yttrium Iron Garnet or “YIG” for example. Even materials such as YIG exhibiting the highest R and small absorption require microns of light-to-MO material interaction length to get reasonable magnetic field resolution. This relatively large spatial requirement, in turn, significantly sacrifices spatial resolution. The Kerr effect, although it provides good spatial resolution, is typically too weak to provide good magnetic resolution for weak or varying magnetic fields.
Enhancement of the MO effect in MOIF is clearly needed. Optimizing the MO layer composition could bring some improvement in MOIF, but it seems doubtful that such improvement would exceed a factor of two since much effort has been expended over several decades of work on materials for such technology as bubble memories. The enhancement of MO Kerr effects near the conditions of SPR excitation have been proposed. See for example Safarov V. I. et al, Physical Review Letters, 73 (26), December 1994. p. 3584–7. Although a strong increase in the MO signal can be realized, this configuration may be hard to adapt for the high spatial resolution required for many imaging and visualization applications. Specifically, the exemplary optical scheme requires a prism and nearly 45 degrees angle of incidence on the MOIF, which may be hard to accomplish in microscopy.
One of the present inventors previously proposed to enhance MO Faraday effects near the condition of surface plasmon resonance (“SPR”) excitation. See Kochergin V. E. et al, JETP Letters, 68 (5), September 1998, p. 400. The experimental structure examined in that article provided a Bi:YIG layer with a thickness of 1.9 micrometers grown on a (111) GGG substrate by liquid phase epitaxy (LPE). A diffraction grating was inscribed on it by ion etching. An Ag layer was deposited on the top of the grating to support the surface plasmons, and was covered by an Au protective layer. The grating depth was 300 nm, which was at least 20 times deeper than it should have been for the maximum effect, so the grating period was unsuitable for normal incidence operation. The end result enhanced the polarization rotation by 6 times over the polarization rotation of un-patterned YIG.
The polarization rotation of the exemplary prior art structure of Kochergin et al as a function of an angle of incidence is given in FIG. 3, which shows experimentally measured enhancement of the MO effect near the conditions of SP excitation. Although the scheme proposed in Kochergin et al has some of the same disadvantages as the Safarov et al proposal, it can be adapted for microscopy applications by choosing the correct grating period. The Kochergin enhancement coefficient was of the same order of magnitude as in Safarov et al, but the absolute value of the observed MO effect was higher by orders of magnitude. However, the Kochergin arrangement relied on a diffraction grating etched into the YIG layer. The diffraction grating structure can induce considerable demagnetization effect and increase coercive force. Therefore, that arrangement is not suitable for certain applications. Accordingly, further improvements and developments are desirable.
Exemplary illustrative arrangements disclosed herein provide an improved MOIF structure with increased magnetic field resolution, spatial resolution and visualization contrast.
One exemplary non-limiting MOIF structure is constructed in the form of multilayer stack containing at least one layer of MO-active material and at least one additional layer whose thickness and/or refractive index is modulated in a predetermined fashion and is not required to be magnetic. The modulation can be made in the form of surface or interface periodical corrugations for example. The corrugations can for example be a one-dimensional diffraction grating, the amplitude and period of which are chosen to maximize the figure of merit of the MOIF structure.
According to a further non-limiting exemplary arrangement, the corrugation can be made in a form of a two-dimensional diffraction grating the period and amplitude of modulation of which are chosen according to the desired performance of the MOIF structure. The corrugation can also be provided by a plurality of superimposed diffraction gratings with equal or different amplitudes according to appropriate design considerations.
According to a further non-limiting implementation, the corrugated layer can be made by self-assembly or deposited by any other method known in the art, of colloidal or particle matter. The colloid or particle sizes may be uniform or random and may have a size and material chosen according to design considerations.
According to a further non-limiting implementation, the corrugation can be made of a self-affine fractal structure formed by the deposition of a thin metallic film under specific conditions in which the thickness is below or near the percolation threshold, or by any other method known to those skilled in the art.
According to a further non-limiting implementation, a MOIF structure is provided that will support at least one optical mode. The propagating mode may be a waveguide mode, a surface mode, a surface plasmon (SP) mode or a hybrid mode for example. The SP mode can be either propagating or localized according to design considerations.
According to a further non-limiting implementation, the MOIF structure can have an antireflection (AR) layer (or multilayer antireflection coating). The AR layer can be provided on the side of the substrate opposite to one having MO-active material disposed thereon. The exemplary AR layer (or AR coating) will suppress unwanted interference between waves reflected by a side or surface of the substrate opposite to a side or surface having an MO-active layer and waves reflected by a reflecting area of the MOIF adjacent to the MO active layer.
According to a further non-limiting implementation, the exemplary MOIF structure may have a reflective layer (or high reflectance multilayer) contiguous with an MO-active layer in order to provide sufficient reflection of the light from the interface.
According to a still further exemplary implementation, the MOIF structure may have a protective layer disposed adjacent to an object to be tested (i.e., at the “device under test” or “DUT” interface of the MOIF structure).
According to a still further exemplary implementation, the MOIF structure may have at least one layer of material chosen to improve the propagation properties of the optical mode(s) supported by the MOIF structure.
The exemplary MOIF structure design described herein can be applied to nondestructive, real-time characterization of magnetic domain structures for technologically important magnetic materials and devices, such as spin-valves, ultra-thin multilayers, granular systems, permanent magnet quality control, integrated circuit (IC) electrical current visualization, magnetic flux visualization, and to the investigation of superconductors, among many other applications The exemplary MOIF structure described herein can find applications in polarized microscopes, laser scanning microscopes, or any other optical method known to those skilled in the art utilizing at least one optical polarizer.